Passivated nanoparticles, method of fabrication thereof, and devices incorporating nanoparticles

ABSTRACT

A plurality of semiconductor nanoparticles having an elementally passivated surface are provided. These nanoparticles are capable of being suspended in water without substantial agglomeration and substantial precipitation on container surfaces for at least 30 days. The method of making the semiconductor nanoparticles includes reacting at least a first reactant and a second reactant in a solution to form the semiconductor nanoparticles in the solution. A first reactant provides a passivating element which binds to dangling bonds on a surface of the nanoparticles to passivate the surface of the nanoparticles. The nanoparticle size can be tuned by etching the nanoparticles located in the solution to a desired size.

This application is a DIV of application Ser. No. 10/233,587 filed Sep.4, 2002, now U.S. Pat. No. 6,906,339, which claims benefit ofprovisional application Ser. No. 60/316,979, filed on Sep. 5, 2001.

FIELD OF THE INVENTION

The present invention is directed generally to semiconductor materialsand more particularly to passivated nanoparticles.

BACKGROUND OF THE INVENTION

Numerous patents and publication have been issued on the fabrication ofsemiconductor nanoparticles (also called “quantum dots” or“nanocrystals”) and their applications in the last few years. Thesesemiconductor nanoparticles are used as a lasing medium in a laser, asfluorescent tags in biological testing methods and as electronicdevices.

A relatively new correlative method for easier manipulation and spatialorganization of the nanoparticles has been proposed in which thenanoparticles are encapsulated in a shell. The shells which encapsulatethe nanoparticles are composed of various organic materials such asPolyvinyl Alcohol (PVA), PMMA, and PPV. Furthermore, semiconductorshells have also been suggested.

For example, U.S. Pat. Nos. 6,225,198 and 5,505,928, incorporated hereinby reference, disclose a method of forming nanoparticles using anorganic surfactant. The process described in the U.S. Pat. No. 6,225,198patent includes providing organic compounds, which are precursors ofGroup II and Group VI elements, in an organic solvent. A hot organicsurfactant mixture is added to the precursor solution. The addition ofthe hot organic surfactant mixture causes precipitation of the II-VIsemiconductor nanoparticles. The surfactants coat the nanoparticles tocontrol the size of the nanoparticles. However, this method isdisadvantageous because it involves the use of a high temperature (above200° C.) process and toxic reactants and surfactants. The resultingnanoparticles are coated with a layer of an organic surfactant and somesurfactant is incorporated into the semiconductor nanoparticles. Theorganic surfactant negatively affects the optical and electricalproperties of the nanoparticles.

In another prior art method, II-VI semiconductor nanoparticles wereencapsulated in a shell comprising a different II-VI semiconductormaterial, as described in U.S. Pat. No. 6,207,229, incorporated hereinby reference. However, the shell also interferes with the optical andelectrical properties of the nanoparticles, decreasing quantumefficiency of the radiation and the production yield of thenanoparticles.

The commercialization of the nanoparticles has also been hampered due tothe high cost of production of the nanoparticles. The methods used forsynthesis are extremely toxic at high temperatures and hence posesignificant safety problems during mass production.

Furthermore, it has been difficult to form nanoparticles of a uniformsize. Some researchers claimed to have formed nanoparticles in asolution having a uniform size based on transmission electron microscopy(TEM) measurements and based on approximating nanoparticle size from theposition of the exciton peak in the absorption spectra of thenanoparticles. However, the present inventor has determined that both ofthese methods do not lead to an accurate determination of nanoparticlesize in the solution.

TEM allows actual observation of a few nanoparticles precipitated on asubstrate from a solution. However, since very few nanoparticles areobserved during each test, the nanoparticle size varies greatly betweenobservations of different nanoparticles from the same solution.Therefore, even if a single TEM measurement shows a few nanoparticles ofa uniform size, this does not correlate to an entire solution ofnanoparticles of a uniform size.

Using the absorption spectra exciton peak position to approximatenanoparticle size is problematic for a different reason. The excitonpeak position does not show whether the individual nanoparticles in asolution are agglomerated into a large cluster. Thus, the size of theindividual nanoparticles that is estimated from the location of theexciton peak in the absorption spectra does not take into account thatthe individual nanoparticles have agglomerated into clusters.

For example, Evident Technologies (www.evidenttech.com) markets EviDots®CdSe nanocrystal test kits. The Evident Technologies website indicatesthat the nanocrystals in these kits are coated with a surfactant and canbe stored unopened in toluene solvent for up to two months. This websitealso implies that the nanoparticles in the test kit solution have auniform size. It appears that the nanoparticle size was approximatedfrom the exciton peak position of the absorption spectra of thesenanoparticles. However, when the present inventor arranged for the sizeof these EviDots® nanoparticles to be measured by a photon correlatedspectroscopy (PCS) method, the results indicated that the nanoparticlesin the solution have agglomerated into large clusters and have anon-uniform size distribution. Thus, while the agglomeratednanoparticles have acceptable optical properties, they have unacceptablemechanical properties due to the agglomeration for uses which require aprecise size distribution, such as for biological marker use.Furthermore, the EviDots® nanoparticles agglomerate into large, visibleclumps and precipitate out of the solution onto the bottom of the vialin less than an hour after the toluene containing vial is unsealed.

BRIEF SUMMARY OF THE INVENTION

A preferred embodiment of the present invention provides a plurality ofsemiconductor nanoparticles having an elementally passivated surface.

Another preferred embodiment of the present invention provides aplurality of semiconductor nanoparticles having an average size betweenabout 2 nm and about 100 nm with a size standard deviation of less than60 percent of the average nanoparticle size determined by photoncorrelated spectroscopy (PCS) method.

Another preferred embodiment of the present invention provides a methodof making semiconductor nanoparticles, comprising forming semiconductornanoparticles of a first size in a solution, and providing an etchingliquid into the solution to etch the semiconductor nanoparticles of thefirst size to a second size smaller than the first size.

Another preferred embodiment of the present invention provides a methodof making semiconductor nanoparticles, comprising reacting at least afirst reactant and a second reactant in a solution to form thesemiconductor nanoparticles in the solution, wherein a first reactantprovides a passivating element which binds to dangling bonds on asurface of the nanoparticles to passivate the surface of thenanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional view of a magnetic data storage mediumaccording to one preferred embodiment of the present invention.

FIG. 2 is a top view of optical data storage medium according to anotherpreferred embodiment of the present invention.

FIG. 3 is a side cross sectional view of an optical cantilever device toanother preferred embodiment of the present invention.

FIG. 4 is a side cross sectional view of an electroluminescent deviceaccording to another preferred embodiment of the present invention.

FIG. 5 is a side cross sectional view of a photodetector according toanother preferred embodiment of the present invention.

FIG. 6 is a schematic illustration of process steps in a method ofmaking nanoparticles according to the preferred embodiments of thepresent invention.

FIGS. 7-11 are PCS spectra from samples made by the method according tothe preferred embodiments of the present invention.

FIG. 12-15 are PCS spectra from samples made by the prior art method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventor has realized that prior art nanoparticlesagglomerate and have a non-uniform size distribution because danglingbonds cause separate nanoparticles to bond together. Therefore, byforming nanoparticles with a passivated surface decreases bondingbetween nanoparticles and thus decreases nanoparticle agglomeration. Apassivated surface comprises surface having passivated dangling bonds,where a passivating element is bound to the dangling bonds. Furthermore,the passivating element reduces or eliminates undesirablereconstructions of atomic positions on the nanoparticle surface whichcause energy levels to appear within the band gap of the nanocrystals.These extra energy levels may lead to decreased light emission atdesired wavelengths and emission and absorption of light at undesiredwavelengths.

It should be noted that a true or “elemental” passivated surface shouldbe distinguished from formation of a semiconductor or organic surfactantshell around a nanoparticle of the prior art. While these prior artnanoparticles with a shell are sometimes referred to as “passivated”,the shell usually does not have the capability of passivating danglingbonds on the surface of the nanoparticle because the lattice constant ofthe shell is different from the lattice constant of the nanoparticles.The lattice mismatch of the shell and nanoparticles would actuallycreate more surface states at the shell/nanoparticle interface, whichdegrades nanoparticle luminescence efficiency. Thus, such prior artnanoparticles that are encapsulated in a shell are not truly“passivated.” Furthermore, nanoparticles coated in a shell are notelementally passivated because an organic or inorganic compound isformed on the nanoparticle, rather than a passivating element which isbound to the dangling bonds.

The elementally passivated nanoparticles can be formed and stored inwater, rather than in an organic solvent as are prior art nanoparticles.Furthermore, in contrast to the prior art nanoparticles, the elementallypassivated nanoparticles do not significantly agglomerate even when thevial with the water solvent is left unsealed.

The present inventor has also realized that nanoparticles with a uniformsize distribution can be formed by first forming large nanoparticleswith a random size distribution and then reducing the size of thenanoparticles to the desired size by etching the larger nanoparticles.By selecting an appropriate type and amount of etching medium, the largenanoparticles can be automatically etched down to a uniform smallersize. In contrast, the prior art nanoparticle formation methods attemptto control the size of the nanoparticles by inhibiting growth ofnanoparticle nuclei in a solution by using a surfactant or a shell.However, it is difficult to control nanoparticle size distribution bythe prior art method because nucleation and growth of nanoparticles isunpredictable and subject to many environmental variables.

The term nanoparticles includes particles having an average size betweenabout 2 and about 100 nm, preferably particles having an average sizebetween about 2 and about 50 nm. Most preferably, the nanoparticlescomprise quantum dots having an average size between about 2 and about10 nm. Preferably, the first standard deviation of the size distributionis 60% or less, preferably 40% or less, most preferably 15 to 30% of theaverage particle size.

In a quantum dot, an electron is confined to a piece of semiconductormaterial small enough such that the volume in which the electron residesis on the order of the wave-volume of the electron itself. Theelectron's formerly continuous spectrum will become discrete, and itsenergy gap will increase. The size of semiconductor volume necessary toachieve perceived electron confinement should be on the order of theBohr radius of the bound state of an electron-hole pair (i.e., anexciton).

The nanoparticles may comprise any material that can form nanoparticles,such as a II-VI, IV-VI, or a III-V semiconductor material. Preferably,the nanoparticles comprise IV-VI or II-VI semiconductor nanoparticles,such as CdS, ZnS, PbS, CdSe, ZnSe, PbSe, ZnTe, PbTe and CdTenanoparticles. Ternary and quaternary semiconductor nanoparticles, suchas CdZnS, CdZnSe, CdZnTe, CdZnTeSe and CdZnSSe, for example, may also beused. Furthermore, semiconductor nanoparticles other than IV-VI or II-VInanoparticles may also be used. These nanoparticles include GaAs, GaP,GaN, InP, InAs, GaAlAs, GaAlP, GaAlN, GalnN, GaAlAsP, GaAlInN, andvarious other II-V materials.

The passivating element may be any element which can bind to andpassivate dangling bonds on the nanoparticle surface. Preferably, thepassivating element comprises one or more elements other than carbon(i.e., an inorganic passivating agent). Most preferably, the passivatingelement is selected from a group consisting of H, S, Se and Te. Sulfuris the most preferred of these passivating elements. Thus, for II-VI andIV-VI semiconductor nanoparticle, by using a nonstoichiometric or excessamount of the Group VI element in the Group VI containing reactantcompound, it is possible to locate the Group VI compound, such as S, Seand Te on the surface of the nanoparticle in such a way as to passivatethe dangling bonds. For III-V semiconductors, an extra reactantcontaining S, Se and Te may be added to passivate the nanoparticlesurface. Furthermore, hydrogen passivating element may be provided byusing a hydrogen containing reactant, such as an ammonia compound,during the formation of the nanoparticles. Thus, a IV-VI or Il-VIsemiconductor nanoparticle passivated by its Group VI element (i.e.,CdS, PbS or ZnS passivated by S, CdTe passivated by Te, etc.) isconsidered to be self passivated.

For example, for sulfide containing IV-VI or II-VI semiconductornanoparticles (i.e., CdS, PbS, ZnS and compounds thereof), thepassivating element preferably comprises S and/or H. Sulfur is mostpreferred. For selenide containing IV-VI or II-VI semiconductornanoparticles (i.e., CdSe, PbSe, ZnSe and compounds thereof), thepassivating element preferably comprises Se and/or H. For telluridecontaining IV-VI or II-VI semiconductor nanoparticles (i.e., CdTe, PbTe,ZnTe and compounds thereof), the passivating element preferablycomprises Te and/or H. Alternatively, sulfur is used as the passivatingelement for the selenide or telluride nanoparticles.

The passivated nanoparticles are capable of being suspended in waterwithout substantial agglomeration and substantial precipitation oncontainer surfaces for at least 30 days, preferably for at least 90days. This means that at least 70%, preferably 95%, most preferably over99% of the nanoparticles are suspended in water without agglomeratingand precipitating on the bottom and walls of the container. It should benoted that the passivated nanoparticles may also be suspended in liquidsother than water without substantial agglomeration and substantialprecipitation on container surfaces for at least 30 days, preferably forat least 90 days.

The passivated nanoparticles may be fabricated using an environmentallyfriendly, non-toxic, low temperature (room temperature or close to roomtemperature) process. Due to its simple chemistry, the nanoparticles canbe fabricated in large quantities from water based solutions. Thepassivated nanoparticles synthesized by the preferred method of thepresent invention are comparable or better in terms of crystallinequality than the prior art nanoparticles and are significantly morestable (i.e., in terms of agglomeration and precipitation form asolution) than the prior art nanoparticles.

A preferred method of making passivated semiconductor nanoparticlescomprises reacting at least a first reactant and a second reactant in asolution to form the semiconductor nanoparticles in the solution. Thefirst reactant provides a passivating element which binds to danglingbonds on a surface of the nanoparticles to passivate the surface of thenanoparticles.

The semiconductor nanoparticles can be formed by reacting two compoundsin a solution using a century old room temperature solution chemistry.Preferably, one of the reactants is an ammonium compound, such asammonium sulfide. For example, to form PbS nanoparticles, the twopreferred reactants can be (NH₄)₂S_(1+x) and PbO. PbO is preferably insolid form and (NH₄)₂S_(1+x) is preferably in liquid form. The “x” in(NH₄)₂S_(1+x) means that an excess (i.e., nonstoichiometric) amount ofsulfur is provided in this reactant. When these reactants are mixedtogether, PbS precipitates out of the solution in solid form and theammonia evaporates from the liquid according to the following reaction:PbO(s)+(NH₄)₂S_(1+x)(I)→PbS(s)+2NH₃(g)+H₂O (I)  (1)

While this reaction has been known for over one hundred years, thepresent inventor believes that it has only been used to form bulk PbSpowder, and not for nanoparticle fabrication. Thus, there was norecognition in the prior art that the reaction conditions could becontrolled to form nanoparticles rather than a bulk PbS powder. Itshould be noted that the nanoparticles may have a different color thanthe bulk PbS powder. The nanoparticle color depends on their size, sincethe band gap of the nanoparticles and thus the absorption cut offwavelength is a function of the nanoparticle size. The nanoparticleshave a range of color, such as red, orange, yellow, etc. In contrast,the bulk PbS powder appears black to a human observer since it absorbsall light.

There are several different methods to form nanoparticles rather thanbulk powder by controlling the reaction conditions to obtain a desiredaverage particle size. One method of controlling the reaction conditionscomprises diluting at least one reactant with water. For example, the(NH₄)₂S_(1+x) reactant may be diluted with water prior to mixing it withPbO. Another method of controlling the reaction conditions comprisescontrolling the pH of the solution by adding an acid, such as HCl to thesolution. A third method of controlling the reaction conditionscomprises selecting a solid second reactant powder, such as PbO powder,having a predetermined average particle size. Any two, and preferablyall three methods are used in combination to obtain nanoparticles ratherthan bulk powder.

Preferably, the addition of the acid to the solution etches thenanoparticles to the desired size. Preferably, the acid compriseshydrochloric acid (HCl). Using etchants to etch nanoparticles insolution depends on the reaction chemistry with the other reagentspresent in the solution. The present inventor has attempted to etch PbSsemiconductor nanoparticles with a variety of acids, such as HCl, HNO₃,HF, H₂O₂, CH₃COOH and combinations of the above. While etching of thePbS sulfide nanoparticles occurred with more than one etchant, thepresent inventor found that the etching rates for acids other than HClwere not uniform. Moreover, reaction of the etchants other than HCl withthe chemical by-products or reagents present in the solution resulted information of undesired precipitates, which leads to difficulties inextracting the nanoparticles from the solution and causes undesiredabsorption of nanoparticles on the precipitates. However, the use ofhydrochloric acid, HCl, to etch the IV-VI sulfide nanoparticles, such asPbS, resulted in uniform etch rates and absence of undesiredprecipitates. It should be noted that the present invention is notlimited to the use of HCl, and that other etchants may be used for othernanoparticle compositions.

Therefore, nanoparticles with a large average size and a non-uniformsize distribution may be formed in a solution first. Then, the acid,such as HCl, is added to the solution and the solution is agitated, suchas by a magnetic stirrer. The acid reduces the size of the nanoparticlesto the desired size by etching the nanoparticles. Thus, the etching“tunes” the nanoparticles to a desired size. The general reactionchemistries for the etching step of PbS and CdS nanoparticles are shownbelow:PbS+H₂O+2HCl→PbCl₂+H₂S+H₂O  (2)CdS+H₂O+2HCl→CdCl₂+H₂S+H₂O  (3)

A model depicting the size tuning of PbS nanoparticles is shown in FIG.6. First, PbS nanoparticles with a large size are formed in a watersolution. Then HCl is added to the solution (box 61 in FIG. 6). HClreacts with the PbS nanoparticles and forms PbCl₂ and H₂S (box 63 inFIG. 6). The etched PbS nanoparticles with the smaller and uniform sizeremain in the water. PbCl₂ remains dissolved in water while H₂S volatilegas escapes from the solution (box 65 in FIG. 6).

The excess passivating element in the solution, such as sulfur, thenrepassivates the surface of the etched nanoparticles. By selecting anappropriate type and amount of etching medium, the large nanoparticlescan be automatically etched down to a uniform smaller size. If the acidconcentration is the solution exceeds the desired amount then thenanoparticles are completely dissolved. The amount of acid is preferablyselected based on the amount of the solid Group II reactant added to thesolution. Thus, since the amount of Group II reactant, such as PbO orPbCl₂, added to the solution is known, the proper amount of acid to beadded to the solution can be readily determined.

It should be noted that reactants other than PbO and (NH₄)₂S_(1+x) maybe used to form PbS nanoparticles. Group IIB and IVA metal chloridereactants may also be combined with Group VI sodium reactant in a watersolvent to form IV-VI and II-VI nanoparticles and salt.PbCl₂+Na₂S+H₂O→PbS+2NaCl+H₂O  (4)

While a Group VI sodium reactant, such as Na₂S, is preferred other GroupIA-Group VI reactants may be used instead. The water solvent mayoptionally contain a small amount of a Group VI ammonia compound, suchas (NH₄)₂S_(1+x), which is sufficient to passivate the nanoparticles.Alternatively, the Group VI ammonia compound may be omitted and anexcess of the Group VI sodium compound compared to the metal chloridecompound may be used instead to provide sufficient sulfur to passivatethe nanoparticles. Thus, the excess sulfur for passivation can come froma nonstoichiometric sulfur compound and/or from using a larger molaramount of the sulfur compound than the molar amount of the metalcompound. Therefore, a nonstoichiometric sulfur compound is not requiredto obtain sulfur passivation.

As indicated above, II-VI nanoparticles other than PbS may be fabricatedusing similar water based reaction chemistry. Thus, zinc or cadmiumoxide or chloride may be mixed with ammonium sulfide or sodium sulfideto form ZnS and CdS nanoparticles. For example, the following reactionsmay be used to form zinc and cadmium sulfide nanoparticles:CdO(s)+(NH₄)₂S_(1+x)(I)→CdS(s)+2NH₃(g)+H₂O(I)  (5)ZnO(s)+(NH₄)₂S_(1+x)(I)→ZnS(s)+2NH₃(g)+H₂O(I)  (6)CdCl₂+Na₂S+H₂O→CdS+NaCl+H₂O  (7)ZnCl₂+Na₂S+H₂O→ZnS+NaCl+H₂O  (8)

Other reactants, such as metal nitrate, perchlorate, acetate and sulfatecompounds and H₂S and Na₂S₂O₃ sulfur compounds are less preferred, butmay be used if desired. For example, to form CdS nanoparticles,Cd(NO₃)₂, Cd(ClO₄)₂, Cd(CH₃COO)₂, Cd(SO₄)₂ may be used as a cadmiumsource. Alternatively, various other organic reactants, such asthiourea, trioctylphosphine sulfide (TOPS), Cd[NH₂CSNH]₂Cl₂ or Cd(CH₃)₃are also less preferred, but may be used if desired.

Alternatively, selenide and telluride nanoparticles may be formedinstead of sulfide nanoparticles. (NH₄)₂SeO₄ dissolved in water may beused as one reactant and a solid lead, cadmium or zinc compound, such asan oxide compound, may be used as the other reactant to form PbSe, CdSeor ZnSe nanoparticles having a selenide passivated surface. A similarreaction may be carried out for telluride based nanoparticles using(NH₄)₂TeO₄ dissolved in water to form PbTe, CdTe or ZnTe nanoparticles.Selenide and telluride containing nanoparticles may also be formed usinga sodium selenide or telluride source, such as Na₂Se and Na₂Te insteadof Na₂S in formulas (4), (7) and (8) above.

To form III-V semiconductor nanoparticles having a surface passivated byat least one of sulfur and hydrogen, liquid (NH₄)₂S_(1+x) reactant isprovided in addition to Group III and Group V reactants.

Several exemplary methods of making PbS nanoparticles are describedbelow. These methods should not be considered as being limiting on thescope of the invention. In one example, two grams of PbCl₂ powder aredissolved in 100 ml of warm water, preferably 60-70° C. water. Thetemperature of the water controls the dissolution time of the powder.Water at other temperatures, such as at room temperature can be usedinstead. Lower water temperature results in longer dissolution times. Inparallel and separately, two grams of Na₂S powder are dissolved in 100ml of warm water, preferably 60-70° C. water. The initial size of theformed nanoparticles can be tuned by mixing different quantities of theabove two solutions. For example, when 5 ml of PbCl₂ solution is addedto 10 ml of Na₂S solution the nanoparticle sizes are smaller (about 50nm) than when 15 ml of Na₂S solution is used (particle sizes of about120 nm). Thus, the nanoparticle size decreases as the PbCl₂ to Na₂Sratio increases. The solution should be richer in sulfur than lead forpassivating the nanoparticle surfaces. Post synthesis nanoparticle sizetuning is done by adding a dilute solution of HCl: H₂O (1:50 by volumepercent, where 1 ml of HCl is dissolved in 50 ml of H₂O) to the watercontaining the nanoparticles. This HCl: H₂O solution is added to thewater containing the nanoparticles in small amounts, such as in 2 mlamounts, to tune the nanoparticle size.

In another example, 10 grams of PbO powder are dissolved in 100 ml ofwarm (NH₄)₂S_(1+x) aqueous solution, preferably having a temperature of30-40° C. The temperature of the solution controls the dissolution timeof the powder. The (NH₄)₂S solution preferably contains of excess sulfurdue to non-stoichiometry which assists in passivation of thenanoparticles. The initial size of the nanoparticles can be tuned bymixing different quantities of PbO to the sulfur solution. For example,when 5 grams of PbO are added to 100 ml of sulfur solution, thenanoparticle sizes are smaller (about 70 nm) than when 10 grams of PbOare added to 100 ml of the sulfur solution (about 150 nm). Thus, thenanoparticle size increases as the PbO to sulfur solution ratioincreases. Post synthesis nanoparticle size tuning is done by adding adilute solution of HCl: H₂O (1:50 by volume percent, where 1 ml of HClis dissolved in 50 ml of H₂O) to the water containing the nanoparticles.This HCl: H₂O solution is added to the water containing thenanoparticles in small amounts, such as in 2 ml amounts, to tune thenanoparticle size.

To narrow the size distribution, one or more purification or particleseparation steps are preferably performed before or after the chemicalreaction of the reactants. Preferably, the filtering step takes placebefore the reaction. Preferably, the entire amount of Group II or IV andGroup VI reactants are mixed together at the same time to form thenanoparticles, followed by etching the nanoparticles. This providesnanoparticles with a uniform size, such that post etching filteringsteps are not required. However, if the Group VI reactant is slowlyadded to the Group II reactant, then post reaction filtering steps maybe needed.

One such particle separation step comprises centrifuging a containercontaining the solution after reacting the first and the secondreactants (i.e., centrifuging the solution containing the formednanoparticles). Distilled water is added to the sample and thenanoparticles are agitated back into solution in an ultrasonic vibrator.The process of centrifuging and washing may be repeated a plurality oftimes, if desired.

The above solution is then filtered through mesh or filters after thesteps of centrifuging and washing. The mesh or filter can be from madefrom randomly oriented stacks of cellulose, spherical columns ofdielectric materials, polymers, nano-porous media (such as alumina orgraphite).

An alternative method to make nanoparticles with a specific size is todecant the solution by storing it for several hours. A first set ofheavy or large nanoparticles or nanoclusters settle at the bottom of thecontainer. The second set of smaller nanoparticles still located in atop portion of the solution is separated from the first set ofnanoparticles and is removed to a new container from the top of thesolution. This process can be repeated several times to separatenanoparticles with different size. During each successive step, theoriginal reagent solution is diluted with a liquid medium which does notdissolve the nanoparticles, such a water. The decanting step may be usedinstead of or in addition to the centrifuging and filtering steps.

To obtain ultra-pure nanoparticles suitable for various applications, itis desirable to reduce or even eliminate the by-products from thechemical reaction formed during the nanoparticle reaction. Theby-products can be reduced or eliminated by choosing a solvent thatdissolves the by-products, but not the nanoparticles. For example,ammonia dissolves in water, but PbS does not. Hence ultra-high puritywater can be used as a solvent for repeated dissolution of theby-products. Thus, the by-products are reduced or eliminated during thecentrifuging and washing steps since the ammonia is dissolved in water.A solvent other than water may be used, depending on the reaction andthe by-products that must be dissolved.

The chemical reaction may be carried out at low temperature, such asbelow 100° C., preferably below 70° C., such as between room temperature(about 25° C.) and 50° C. Furthermore, non-toxic, inorganic reactantsare used, such as PbO, PbCl₂, Na₂S and ammonium sulfide. Since thereaction is carried out in water from inorganic reactants, the processis inexpensive. The process does not require the use of a surfactant(although one may be used if desired) and a shell is not required to beformed around the nanoparticles as in the prior art. Furthermore, sincethe core/shell approach is not necessary, a large volume of thenanoparticles may be produced from the reactants with a high yield.Since the passivated nanoparticles are stable in solution, such aswater, for several months, they can be easily transported from themanufacturing location to the location where the nanoparticles areplaced into an article of manufacture. Furthermore, the passivatednanoparticles may be stored in water or other fluid for one month ormore before they are placed into an article of manufacture.

After fabrication, storage and/or transportation, the nanoparticles aresuspended in fluid, such as a solution, suspension or mixture. Suitablesolutions can be water as well as organic solvents such as acetone,methanol, toluene, alcohol and polyvinyl alcohol. Alternatively, thenanoparticles are located or deposited on a solid substrate or in asolid matrix. Suitable solid matrices can be glass, ceramic, cloth,leather, plastic, rubber, semiconductor or metal. The fluid or solidcomprises an article of manufacture which is suitable for a certain use.

The following preferred embodiments provide preferred articles ofmanufacture which incorporate the nanoparticles. It should be noted thatwhile these articles of manufacture preferably contain the semiconductornanoparticles where a passivating element passivates dangling bonds onthe nanoparticle surface, as described above, they may also containsemiconductor nanoparticles which are made by any other method,including by the prior art methods. In the first four preferredembodiments, the nanoparticles are provided into a fluid.

In a first preferred embodiment, the semiconductor nanoparticles areplaced into a polishing slurry. The nanoparticles are dispersed in thepolishing slurry fluid. Since the nanoparticles have a very high surfacehardness due to their small size, the nanoparticles function as anabrasive medium in the slurry fluid. If desired, another abrasive mediumin addition to the nanoparticles may be added to the slurry. Thepolishing slurry may be used to polish any industrial articles, such asmetal or ceramics. Preferably, the slurry is adapted to be used in achemical-mechanical polishing apparatus used to polish semiconductorwafers and devices. In this case, in addition to the nanoparticles, theslurry also contains a chemical which chemically removes a portion ofthe semiconductor wafers and devices.

In a second preferred embodiment, the semiconductor nanoparticles areplaced into a paint. The nanoparticles are dispersed in the liquid baseof the paint. Since the nanoparticles have a uniform size distribution,they provide a substantially uniform color to the paint. In a preferredaspect of the second embodiment, the liquid paint base is selected suchthat it evaporates after being coated on a surface, such as a wall,ceiling or floor. After the liquid base evaporates, a layer ofsemiconductor nanoparticles is left on the surface such that thenanoparticles provide a color to the surface. The nanoparticles are verystrongly adhered to the surface due to their small size. Thenanoparticles are almost impossible to remove by physical means, such asbrushes, paint knives or scrubbers, since the nanoparticle size issmaller than the grooves present in the surfaces of the brushes, paintknives or scrubbers. Thus, a chemical method, such as acid etching, isrequired to remove the nanoparticles from a surface. Therefore, thenanoparticle containing paint is especially adapted to function as aprotective paint, such as a rust inhibiting primer paint (which isprovided under a conventional paint layer) or a top coat paint (which isprovided over a layer of conventional paint). Thus, the nanoparticlecontaining paint is especially adapted to coat outdoor structures, suchas bridges, fences and buildings, since it adheres much better tosurfaces than the conventional paints, primers and top coats.

In a third preferred embodiment, the semiconductor nanoparticles areplaced into an ink. The nanoparticles are dispersed in a liquid ink. Asdescribed above, the nanoparticles can provide a substantially uniformcolor to a liquid. Thus, by placing the nanoparticles into a ink, oncethe ink dries and the liquid base evaporates, an image is formed from alayer of nanoparticles. This image will have a very high adhesion to thesurface on which it is printed. The ink may comprise computer printer(i.e., ink jet printer, etc.) ink, printing press ink, pen ink or tattooink.

In a fourth preferred embodiment, the semiconductor nanoparticles areplaced into cleaning composition. The nanoparticles are dispersed in thecleaning fluid. Since the nanoparticles have a high surface hardness,they add a significant scrubbing power to the cleaning fluid. Thecleaning fluid may comprise any industrial cleaning fluid, such as asurface cleaning/scrubbing fluid or a pipe cleaning fluid.

In the first four preferred embodiments, the nanoparticles are providedinto a fluid. In the following preferred embodiments, the nanoparticlesare provided onto a surface of a solid material.

In the fifth preferred embodiment, the nanoparticles comprise a hardnessor wear resistant coating located on at least a portion of a device. Thedevice may be any device in which a hardness or wear resistant coatingis desired. For example, the device may be a tool (such as a screwdriveror saw blade), a drill bit, a turbine blade, a gear or a cuttingapparatus. Since the nanoparticles have a high surface hardness and avery strong adhesion to a substrate, a layer of semiconductornanoparticles provides an ideal hardness or wear resistant coating for adevice. The coating may be formed by providing a fluid containing thenanoparticles and then evaporating or otherwise removing the fluid toleave a layer of nanoparticles on the device surface.

In the sixth preferred embodiment, the nanoparticles comprise a moisturebarrier layer located on at least one surface of an article ofmanufacture. The moisture barrier layer has few or no pores for water ormoisture to seep through the layer because the layer comprises aplurality of small size nanoparticles contacting each other. The size ofthe individual nanoparticles is much smaller than the size of a drop ofmoisture. Thus, a continuous layer of nanoparticles will resistpenetration of moisture. The article of manufacture containing thenanoparticles may be apparel (i.e., coats, pants, etc. made of cloth orleather) or footwear (made of leather, cloth, rubber or artificialleather). Alternatively, the article of manufacture could comprise anedifice, such as a bridge, building, tent, sculpture, etc. For example,since the nanoparticle layer has a higher adhesion to a structure thanconventional moisture barrier paint, using the nanoparticle moisturebarrier would reduce or eliminate the requirement that the moisturebarrier be the reapplied every few years (as is currently done withbridges). The moisture barrier layer may be deposited by providing afluid containing the nanoparticles and then evaporating or otherwiseremoving the fluid to leave a layer of nanoparticles on the articlesurface. Preferably, the layer is formed on an outer surface of thearticle. If desired, the nanoparticle material could be selected whichabsorbs sunlight and generates heat when exposed to sunlight (i.e., CdTenanoparticles). Alternatively, the material may be selected which trapsheat emitted by a human body.

In a seventh preferred embodiment, the nanoparticles are provided in acomposite ultra low porosity material. Preferably, such a material has aporosity below 10 volume percent, most preferably below 5 volumepercent. The composite material comprises a solid matrix material andthe nanoparticles incorporated into the matrix. The composite materialmay be formed by mixing a matrix material powder and nanoparticle powdertogether and then compressing the mixed powder to form a compositematerial. Since the nanoparticles have a small size, they occupy thepores in the matrix material to form an ultra low porosity compositematerial. The matrix material may comprise ceramic, glass, metal,plastic or semiconductor materials. The ultra low porosity material maybe used as a sealant, such as a tire sealant. Alternatively, thecomposite material may be used as a filler in industrial and medicalapplications.

In an eighth preferred embodiment, the nanoparticles are provided in afilter. A nanoparticle powder may be compressed to form the filter.Alternatively, the nanoparticles may be added to a solid matrix materialto form the filter. Since the nanoparticles have a small size,compressed nanoparticles or nanoparticles in a matrix have a lowporosity. Thus, the nanoparticle filter has a very fine “mesh” and isable to filter very small particles. The porosity of the filter isgreater than the porosity of the ultra low porosity material of theprevious embodiment. Preferably, the filter is used to filter a liquidcontaining very small solid particles. The liquid containing theparticles is poured through the filter, which traps particles above apredetermined size.

In a ninth preferred embodiment, the nanoparticles are provided in acomposite high strength structural material. Since the nanoparticleshave a high surface hardness and low porosity, the nanoparticles may beincorporated into a composite structural material having a solid matrixand nanoparticles dispersed in the matrix. The matrix material maycomprise ceramic, glass, metal or plastic. The structural material maybe used in buildings as supporting columns and walls and in bridges asthe roadway and as supporting columns. The structural material may alsobe used to form parts of machinery and vehicles, such as cars andtrucks.

In a tenth preferred embodiment, the nanoparticles are provided in anenvironmental sensor. The environmental sensor includes a radiationsource, such as a lamp or laser, and a matrix material containing thenanoparticles. The matrix material may comprise liquid, gas or solidmaterial. The sensor is exposed to an outside medium which affects thelight emitting properties of the nanoparticles. For example, the sensormay comprise a pollution sensor which is exposed to atmosphere. Theamount of pollution in the atmosphere affects the microenvironment ofthe nanoparticles, which in turn affects their radiation emissioncharacteristics. The nanoparticles are irradiated with radiation, suchas visible light or UV or IR radiation, from the radiation source. Theradiation emitted and/or absorbed by the nanoparticles is detected by adetector. A computer then determines the amount of pollution present inthe atmosphere based on the detected radiation using a standardalgorithm. The sensor may also be used to sense gas components andcompositions other than the amount of pollution in the atmosphere.

The semiconductor nanoparticles may also be used in lightingapplications. The eleventh through the thirteenth embodiments describethe use of the semiconductor nanoparticles in lighting applications.

In the eleventh preferred embodiment, passivated nanoparticles are usedas a light emitting medium in a solid state light emitting device, suchas a laser or a light emitting diode. In these applications, a currentor voltage is provided to the passivated nanoparticles from a current orvoltage source. The current or voltage causes the passivatednanoparticles to emit light, UV or IR radiation, depending on thenanoparticle material and size.

In the twelfth preferred embodiment, semiconductor nanoparticles areused to provide support for organic light emitting material in anorganic light emitting diode. An organic light emitting diode containsan organic light emitting material between two electrodes. The organiclight emitting material emits light when current or voltage is appliedbetween the electrodes. The light emitting organic material may be apolymer material or small dye molecules. Both of these organic materialshave poor structural characteristics and impact resistance, which lowersthe robustness of the organic light emitting diodes. However, theseorganic light emitting materials may be incorporated in a matrix ofnanoparticles which provides the desired structural characteristics andimpact resistance. Since the nanoparticles have the same or smaller sizethan the dye or polymer molecules, the nanoparticles do not interferewith the light emitting characteristics of the diode.

In the thirteenth preferred embodiment, semiconductor nanoparticles areused in a fluorescent lamp in place of a phosphor. In a conventionalfluorescent lamp, a phosphor is coated on an inner surface of a shell ofthe lamp. The phosphor absorbs UV radiation emitted by a radiationsource, such as mercury gas located in the lamp shell, and emits visiblelight. Since the semiconductor nanoparticles have the ability to absorbUV radiation emitted by the radiation source and to emit visible light,these nanoparticles may be located on at least one surface of the lampshell. Preferably, the layer of nanoparticles coated on the lamp shellcontains nanoparticles which emit different color light, such that thecombined light output of the nanoparticles appears as white light to ahuman observer. For example, the different color light emission may beobtained by mixing nanoparticles having a different size and/ornanoparticles of different semiconductor materials.

The semiconductor nanoparticles may also be used in magnetic datastorage applications. The fourteenth and fifteenth preferred embodimentsdescribe the use of the semiconductor nanoparticles in magnetic datastorage applications.

In the fourteenth preferred embodiment, the semiconductor nanoparticlesare used in a magnetic data storage device. This device includes amagnetic field source, such as a magnet, a data storage mediumcomprising the nanoparticles, a photodetector. A light source is used toilluminate the nanoparticles. The magnetic field source selectivelyapplies a localized magnetic field source to a portion of the datastorage medium. The application of the magnetic field causes thenanoparticles exposed to the field to change their light or radiationemission characteristics or to quench emission of light or radiation alltogether. The photodetector detects radiation emitted from thenanoparticles in response to the application of a magnetic field by themagnetic field source.

In the fifteenth preferred embodiment, the semiconductor nanoparticlesare used in a magnetic storage medium containing a magnetic material.The magnetic material may be any magnetic material which can store databy the alignment of the directions of the spins in the material. Suchmagnetic materials include, for example, cobalt alloys, such as CoPt,CoCr, CoPtCr, CoPtCrB, CoCrTa and iron alloys, such as FePt and FePd. Inone preferred aspect of the fifteenth embodiment, the nanoparticles 11are randomly mixed throughout a layer of magnetic material 13 formed ona substrate 15, as shown in FIG. 1. The substrate 15 may be glass,quartz, plastic, semiconductor or ceramic. The randomly dispersednanoparticles are located within the magnetic domains in the magneticmaterial. The domains are separated by the domain walls. A few domainwalls are shown by lines 17 in the close up of area “A” in FIG. 1. Thedispersed nanoparticles form barrier layers 19 within the domains. Thebarrier layers form domain walls in the magnetic material. Therefore,the addition of the nanoparticles has the effect of subdividing thedomains in the magnetic material into a plurality of “subdomains” eachof which is capable of storing one bit of data (shown as spin arrows inFIG. 1). Thus, the addition of the nanoparticles increases the datastorage density of the magnetic material by decreasing the domain sizein the magnetic material.

In a second preferred aspect of the fifteenth embodiment, the magneticstorage medium comprises a substrate containing the semiconductornanoparticles doped with atoms of the magnetic material. Eachnanoparticle is adapted to store one bit of data. Thus, smallnanoparticles of magnetic material are encapsulated in the semiconductornanoparticles. In this case, the size of one bit of data storage is onlyas big as the semiconductor nanoparticle. The magnetic nanoparticles maybe doped into the semiconductor nanoparticles using any known dopingtechniques, such as solid, liquid or gas phase diffusion, ionimplantation or co-deposition of the semiconductor and magneticnanoparticles. Alternatively, the magnetic nanoparticles may beencapsulated within the semiconductor nanoparticles by a plasma arcdischarge treatment of semiconductor nanoparticles in contact withmagnetic nanoparticles. Similar methods have been previously disclosedfor encapsulating magnetic particles in carbon and buckytube shells (seeU.S. Pat. Nos. 5,549,973, 5,456,986 and 5,547,748, incorporated hereinby reference).

In the sixteenth preferred embodiment, the nanoparticles are used in anoptical data storage medium, as shown in FIG. 2. Clusters ofnanoparticles 21 are arranged in predetermined patterns on a substrate25, such that first areas 27 of the substrate 25 contain thenanoparticles 21 while the second areas 29 of the substrate 25 do notcontain the nanoparticles 21. The nanoparticles 21 in a solution may beselectively dispensed from an ink jet printer or other microdispenser toareas 27 on the substrate. After the solvent evaporates, a cluster ofnanoparticles remains in areas 27. The substrate 25 may be a glass,quartz, plastic, semiconductor or ceramic substrate. Preferably, thesubstrate 25 is shaped as a disk, similar to a CD. The data from thestorage medium is read similar to a CD, by scanning the medium with alaser or other radiation source. The nanoparticles 21 reflect and/oremit light or radiation differently than the exposed substrate areas 29.Therefore, when the substrate is scanned by a laser, a different amountand/or wavelength of radiation is detected from areas 27 than areas 29by a photodetector. Thus, areas 27 correspond to a “1” data value, whileareas 29 correspond to a “0” data value, or vise-versa (i.e., eachcluster of nanoparticles 21 is a bit of data). Therefore, thenanoparticles 21 function similar to bumps in a conventional CD or as amaterial of a first phase in a phase change optical disk. The areas 27may be arranged in tracks or sectors similar to a CD for ease of dataread out.

The optical data storage medium described above may be used incombination with an optical system of the seventeenth preferredembodiment. The optical system 30 includes at least one microcantilever35 and light emitting nanoparticles 31 located on a tip of the at leastone microcantilever, as shown in FIG. 3. The microcantilever 35 may bean atomic force microscope (AFM) microcantilever or a similarmicrocantilever that is not part of an AFM. For example, themicrocantilever 35 may be conductive or contain conductive leads orwires which provide current or voltage to the nanoparticles to causethem to emit light or radiation. The base 33 of the microcantilever isconnected to a voltage or current source. The microcantilever 35 may bescanned over the substrate 25 containing the semiconductor nanoparticles21 of the previous embodiment. The light emitting nanoparticles 31 onthe cantilever irradiate the substrate 25, and the emitted and/orreflected light is detected by a photodetector and analyzed by acomputer to read out the data. Of course, the optical system 30 may beused to read data from a conventional CD or phase change optical diskrather than from the medium of the previous embodiment. Furthermore, oneor more microcantilevers 35 may be incorporated into an AFM to studysurfaces of materials. In this case, the AFM may be used to study theinteraction of light or radiation emitted by the nanoparticles 31 andthe surface being studied.

In the eighteenth through the twenty first preferred embodiments, thesemiconductor nanoparticles are used in an optoelectronic component.

In the eighteenth preferred embodiment, the light emitting nanoparticlesare used in an optical switch. In the switch, the light emittingnanoparticles are arranged on a substrate and are connected to a voltageor current source which provides the voltage or current for the light(or radiation) emission. A source of magnetic field, such as a magnet,is provided adjacent to the nanoparticles. When the magnet is turned on,it extinguishes radiation emitted by the nanoparticles.

In the nineteenth preferred embodiment, the passivated semiconductornanoparticles are used in an electroluminescent device, such as theelectroluminescent device illustrated in U.S. Pat. No. 5,537,000,incorporated herein by reference. The electroluminescent device 40includes a substrate 45, a hole injection layer 46, a hole transportlayer 47, an electron transport layer 41 and an electron injection layer48, as illustrated in FIG. 4. An voltage is applied between layers 46and 48. The voltage generates holes in layer 46 and electrons in layer48. The holes and electrons travel through layers 47 and 41 andrecombine to emit light. Depending on the applied voltage, therecombination occurs either in layer 41 to emit red light or in layer 47to emit green light. The electron transport layer 41 comprises a layerof passivated semiconductor nanoparticles, such as II-VI nanoparticles.The hole injection layer 46 comprises a conductive electrode, such asindium tin oxide. The hole transport layer 47 comprises an organicpolymer material, such as poly-p(paraphenelyne). The electron injectionlayer 48 is a metal or heavily doped semiconductor electrode, such as aMg, Ca, Sr or Ba electrode.

In a twentieth preferred embodiment of the present invention, thesemiconductor nanoparticles are used in a photodetector 50, such as aphotodetector described in U.S. Pat. No. 6,239,449, incorporated hereinby reference. As shown in FIG. 5, the photodetector is formed on asubstrate 55. A first heavily doped contact layer 52 is formed on thesubstrate. A first barrier layer 53 is formed on the contact layer 52.One or more nanoparticle layers 51 are formed on the barrier layer 53. Asecond barrier layer 54 is formed on the nanoparticle layer(s) 51. Asecond heavily doped contact layer 56 is formed on the second barrierlayer 54. Electrodes 57 and 58 are formed in contact with the contactlayers 52, 56. The barrier layers 53, 54 are doped to provide chargecarriers and for conductivity. The barrier layers 53, 54 have a higherband gap than the nanoparticles 51. Incident light or radiation excitescharge carriers (i.e., electrons or holes) in the nanoparticles to anenergy higher than the energy of the bandgap of the barrier layers 53,54. This causes a current to flow through the photodetector 50 from theemitter electrode to a collector electrode in response to the incidentlight or radiation with the help of an external voltage applied betweenthe electrodes.

In a twenty first preferred embodiment, the nanoparticles are used in atransmission grating. The nanoparticles are arranged on a transparentsubstrate in a form of a grating. Since the nanoparticles have a verysmall size, the grating may be formed with a period smaller than thewavelength of light or radiation that will be transmitted through thegrating. Such gratings may be used in waveplates, polarizers or phasemodulators. The gratings may be formed by patterning the nanoparticleson the substrate using submicron optical, x-ray or electron beamlithography or by placing individual nanoparticles on the substrateusing an AFM or a scanning tunneling electron microscope.

In a twenty second preferred embodiment, the semiconductor nanoparticlesare used in an optical filter. The optical filter may comprise a glass,plastic or ceramic transparent matrix with interdispered nanoparticles.Since the nanoparticles absorb a radiation having a wavelength greaterthan a cutoff wavelength based on the material and size of thenanoparticles, the filter may be tailored to filter a particular rangeof light or UV radiation wavelengths depending on the material and sizeof the nanoparticles. Furthermore, the nanoparticles may be used toprovide a color to a particular solid material, such as stained orcolored glass.

In the twenty third preferred embodiment, the passivated nanoparticlesare used in electronic devices, such as transistors, resistors, diodes,and other nanodevices. For example, the nanoparticles may be used in asingle electron transistor, as described in U.S. Pat. No. 6,057,556,incorporated herein by reference. The nanoparticles are located on asubstrate between a source and a drain electrode. The nanoparticlescomprise a channel of the single electron transistor. A plurality ofnanoscale gate electrodes are provided over or adjacent to thenanoparticles. This device functions on the principle of controlledcorrelated single electron tunneling between the source and drainelectrodes through the potential barriers between the nanoparticles. Asingle electron gate circuit can be constructed using this device, wherelogical “1” and “0” are identified by the presence or absence of anelectron.

An example of a nanodevice array is a chip architecture termed cellularautomata. With this architecture, the processor portion of the IC ismade up of multiple cells. Each of the cells contains a relatively smallnumber of devices, which communicate only with their nearest-neighborcells. This architectural approach eliminates the need for longintercellular connections, which ultimately put a ceiling on the fastestprocessing capabilities of an electronic chip. Each cell would consistof approximately five passivated semiconductor nanoparticles or quantumdots.

In the twenty fourth preferred embodiment, the nanoparticles are used asa code or a tag. For example, the nanoparticles may be fashioned into aminiature bar code by AFM, STM or lithography. This bar code may beformed on small items, such as integrated circuits, and may be read by aminiature bar code reader. Of course the code may have symbols otherthan bars. In another example, the nanoparticles may be used as a tag(i.e., where the nanoparticles are not formed into a particular shape).Since a small amount of the nanoparticles is invisible to the human eye,the nanoparticle code or tag may be added to an item which must beauthenticated, such as currency, a credit card, an identification cardor a valuable object. To authenticate the item, the presence of thenanoparticles on or in the item is detected by a microscope or by anoptical detector. Furthermore, nanoparticles of a certain size whichemit a particular wavelength of light may be used to distinguishdifferent objects. Combinations of different nanoparticle sizes whichemit a combination of different wavelengths may be used to emit anoptical code for more precise identification of the item.

In the twenty fifth preferred embodiment, the passivated nanoparticlesare used as sensor probes. For example, a sensor probe may be formed bybonding nanoparticles to affinity molecules using linking agents, asdescribed in U.S. Pat. Nos. 6,207,392, 6,114,038 and 5,990,479,incorporated herein by reference. The affinity molecules are capable ofselectively bonding with a predetermined biological or other substance.In response to an application of energy, the nanoparticles emit light orradiation which is detected by a detector. Thus, the presence, locationand/or properties of the predetermined substance bound to the affinitymolecule may be determined. The linking agents may be polymerizablematerials, such as N-(3-aminopropyl)3-mercapto-benzamide. The affinitymolecules, such as antibodies, are capable of selectively binding to thepredetermined biological substance being detected, such as a particularantigen, etc.

The specific examples of PbS nanoparticles made according to the methodsof the preferred embodiments of the present invention and comparativeexamples using prior art CdSe EvidDots® nanoparticles will now bedescribed. These specific examples are provided for illustration onlyand should not be considered limiting on the scope of the invention.

EXAMPLE 1

PbS nanoparticles according to the preferred embodiments of the presentinvention were prepared by the following method. 2 grams of PbCl₂ powderwere dissolved in 100 ml of warm water at 60-70° C. 2 grams of Na₂Spowder were separately dissolved in 100 ml of warm water (60-70° C.).Both solutions were filtered using 20 nm pore filter paper. 5 ml of theabove PbCl₂ solution was added to 15 ml of above Na₂S solution and mixedin an ultrasonic vibrator.

The nanoparticle containing solution was analyzed by photon correlatedspectroscopy (PCS) using the N4 Plus Submicron Particle Analyzer made byBeckman Coulter, Inc. PCS is a dynamic light scattering (DLS) opticalcharacterization technique which determines particle size based onBrownian motion of particles in a solution. In PCS, measurements ofparticle size are based on characterization of the time scale offluctuations of the intensity of laser light scattered from thenanoparticle containing solution. The fluctuations result from aninterference pattern modulated by the Brownian motion of thenanoparticles in the solution. The fluctuating laser light signal istransformed into an electrical signal by a photomultiplier. Othersimilar dynamic light scattering spectroscopy systems which measure thesize of particles in the solution are produced by other companies. Forexample, one such system is the High Performance Particle Size withNon-Invasive Back Scatter (NIBS™) optics characterization systemproduced by Malvern Instruments.

The PCS spectra of the solution containing the nanoparticles is shown inFIG. 7. The nanoparticles had a 114.2 nm average size with a standarddeviation of 27.8 nm.

EXAMPLE 2

In this example, the nanoparticles of example 1 were etched with dilutedHCl. 1 ml of HCl was dissolved in 50 ml of H₂O. 2 ml of the abovesolution was added to the nanoparticle containing solution of example 1.The PCS spectra of the solution containing the etched nanoparticles isshown in FIG. 8. The nanoparticles had a 53.3 nm average size with astandard deviation of 20.8 nm. Thus, adding HCl to the solution reducedthe nanoparticle size without completely dissolving the nanoparticles.

EXAMPLE 3

In this example, the nanoparticles of example 2 were etched withadditional diluted HCl. 1 ml of HCl was dissolved in 50 ml of H₂O. 2 mlof the above solution was added to the nanoparticle containing solutionof example 2. The PCS spectra of the solution containing the etchednanoparticles is shown in FIG. 9. The nanoparticles had a 17.6 nmaverage size with a standard deviation of 9.1 nm.

EXAMPLE 4

In this example, the nanoparticles of example 3 were etched withadditional diluted HCl. 1 ml of HCl was dissolved in 50 ml of H₂O. 2 mlof the above solution was added to the nanoparticle containing solutionof example 3. The PCS spectra of the solution containing the etchednanoparticles is shown in FIG. 10. The nanoparticles had a 4.5 nmaverage size with a standard deviation of 2.5 nm.

EXAMPLE 5

In this example, the nanoparticles of example 4 were etched withadditional diluted HCl. 1 ml of HCl was dissolved in 50 ml of H₂O. 2 mlof the above solution was added to the nanoparticle containing solutionof example 4. The PCS spectra of the solution containing the etchednanoparticles is shown in FIG. 11. The nanoparticles had a 2.0 nmaverage size with a standard deviation of 0.3 nm (main peak in FIG. 11).

COMPARATIVE EXAMPLE 1

EviDots® CdSe nanocrystals from a test kit supplied by EvidentTechnologies (www.evidenttech.com) were examined by PCS. Thenanocrystals were located in an unopened container containing toluenesolvent during PCS testing. The product literature accompanying the testkit indicated that the nanoparticles had an average size of 2.8 nm thatwas apparently determined from exciton peak position in absorptionspectra. The nanoparticle solution had a green color.

The PCS spectra of the solution containing the nanoparticles is shown inFIG. 12. The nanoparticles had a 12.2 nm average size with a standarddeviation of 12.6 nm. This is evidence that the 2.8 nm nanoparticlesagglomerated in the toluene solution such that the average agglomeratedcluster size is 12.6 nm. It is believed that the agglomerated clustercomprises a surfactant with individual 2.8 nanoparticles embedded in thecluster in a roughly honeycomb structure. The individual nanoparticlesin the cluster are separated from each other by the surfactant. Thesolution appears green because each individual nanoparticle in theagglomerated cluster is about 2.8 nm.

COMPARATIVE EXAMPLE 2

EviDots® CdSe nanocrystals from a test kit supplied by EvidentTechnologies were examined by PCS. The product literature accompanyingthe test kit indicated that the nanoparticles had an average size of 4nm that was apparently determined from exciton peak position inabsorption spectra. The nanoparticle solution had an orange color.

The PCS spectra of the solution containing the nanoparticles is shown inFIG. 12. The nanoparticles had a 71.2 nm average size with a standarddeviation of 76.6 nm. This is evidence that the 4 nm nanoparticlesagglomerated in the toluene solution such that the average agglomeratedcluster size is 71.6 nm.

COMPARATIVE EXAMPLE 3

EviDots® CdSe nanocrystals from a test kit supplied by EvidentTechnologies were examined by PCS. The product literature accompanyingthe test kit indicated that the nanoparticles had an average size of 3.4nm that was apparently determined from exciton peak position inabsorption spectra. The nanoparticle solution had a yellow color.

The PCS spectra of the solution containing the nanoparticles is shown inFIG. 13. The nanoparticles had a 79.8 nm average size with a standarddeviation of 85.8 nm. This is evidence that the 3.4 nm nanoparticlesagglomerated in the toluene solution such that the average agglomeratedcluster size is 79.8 nm.

COMPARATIVE EXAMPLE 4

EviDots® CdSe nanocrystals from a test kit supplied by EvidentTechnologies were examined by PCS. The product literature accompanyingthe test kit indicated that the nanoparticles had an average size of 4.7nm that was apparently determined from exciton peak position inabsorption spectra. The nanoparticle solution had a red-orange color.

The PCS spectra of the solution containing the nanoparticles is shown inFIG. 14. The nanoparticles had a clear bimodal distribution (two peaks)with a 160.2 nm average size with a standard deviation of 140.6 nm. Thisis evidence that the 4.7 nm nanoparticles agglomerated in the toluenesolution such that the average agglomerated cluster size is 160.2 nm.

Thus, several conclusions can be drawn from the examples and thecomparative examples. First, PCS testing shows that nanoparticles in asolution may be agglomerated into a large cluster. Thus, the size of theindividual nanoparticles that is estimated from the location of theexciton peak in the absorption spectra does not take into account thatthe individual nanoparticles have agglomerated into clusters. It appearsfrom the PCS test that the actual average size of nanoparticle clustermay be five to twenty times larger than the size of the individualnanoparticles estimated from the exciton peak location. Second, themethod of the preferred embodiments of the present invention allowsaverage nanoparticle size to be controlled more precisely than the priorart method and avoids or reduces the agglomeration of nanoparticles intoclusters. Third, the nanoparticles made by the method of the preferredembodiments of the present invention have a much narrower sizedistribution than the prior art nanoparticles. For example, thenanoparticles made by the method of the preferred embodiments of thepresent invention have a size standard deviation between about 15 and 56percent of the average nanoparticle size. In contrast, the prior artnanoparticles have a size standard deviation between about 87 to morethan 100 percent of the average nanoparticle size due to agglomeration.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedrawings and description were chosen in order to explain the principlesof the invention and its practical application. It is intended that thescope of the invention be defined by the claims appended hereto, andtheir equivalents.

All of the publications and patent applications and patents cited inthis specification are herein incorporated in their entirety byreference.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the present invention is not so limited. It willoccur to those of ordinary skill in the art that various modificationsmay be made to the disclosed embodiments and that such modifications areintended to be within the scope of the present invention, which isdefined by the following claims.

1. A method of making semiconductor nanoparticles, comprising: formingsemiconductor nanoparticles of a first size in an aqueous solution; andproviding an etching liquid into the solution to etch the semiconductornanoparticles of the first size to a second size smaller than the firstsize; wherein the solution contains a passivating element which binds todangling bonds on a surface of the nanoparticles to passivate thesurface of the nanoparticles.
 2. The method of claim 1, wherein: theetching liquid comprises hydrochloric acid.
 3. The method of claim 2,wherein the step of forming semiconductor nanoparticles comprisescombining a metal Group IIB or lead compound and a Group VI compound ina solvent comprising water.
 4. The method of claim 3, wherein: the metalcompound comprises a Pb, Zn or Cd compound; and the Group VI compoundcomprises a sulfur compound.
 5. The method of claim 4, wherein: Pb, Znor Cd compound comprises PbO, ZnO or CdO; and the sulfur compoundcomprises ammonium sulfide.
 6. The method of claim 4, wherein: Pb, Zn orCd compound comprises PbCl₂, ZnCl₂ or CdCl₂; and the sulfur compoundcomprises sodium sulfide.
 7. The method of claim 4, wherein the solutioncomprises an excess amount of sulfur which binds to dangling bonds on asurface of the nanoparticles to passivate the surface of thenanoparticles.
 8. The method of claim 7, wherein the sulfur compoundcomprises (NH₄)₂S_(1+x) which provides the excess amount of sulfur whichbinds to the dangling bond.
 9. The method of claim 7, wherein thesolution contains a larger molar amount of sulfur compound than themolar amount of the Pb, Zn or Cd compound to provide the excess amountof sulfur which binds to dangling bonds.
 10. A method of makingsemiconductor nanoparticles, comprising reacting at least a firstreactant and a second reactant in a solution to form the semiconductornanoparticles in the solution, wherein the first reactant provides apassivating element which binds to dangling bonds on a surface of thenanoparticles to passivate the surface of the nanoparticles.
 11. Themethod of claim 10, wherein the semiconductor nanoparticles compriseII-VI, III-V or IV-VI semiconductor nanoparticles.
 12. The method ofclaim 11, wherein: the first reactant comprises (NH₄)₂S_(1+x); thesecond reactant comprises a lead, cadmium or zinc compound; and thenanoparticles comprise PbS, CdS or ZnS nanoparticles having a sulfurpassivated surface.
 13. The method of claim 11, wherein: the firstreactant comprises liquid (NH₄)₂SeO₄, the second reactant comprisessolid a lead, cadmium or zinc compound; and the nanoparticles comprisePbSe, CdSe or ZnSe nanoparticles having a selenide passivated surface.14. The method of claim 11, further comprising providing a thirdreactant comprising a Group V element compound; wherein: the firstreactant comprises liquid (NH₄)₂S_(1+x); the second reactant comprises aGroup III element compound; and the nanoparticles comprise III-Vsemiconductor nanoparticles having a surface passivated by at least oneof sulfur and hydrogen.
 15. The method of claim 10, further comprisingdiluting at least one reactant with water.
 16. The method of claim 10,further comprising adding an acid to the solution.
 17. The method ofclaim 16, wherein the acid comprises HCl.
 18. The method of claim 10,wherein: the passivating element comprises sulfur; the first and thesecond reactants comprise inorganic reactants; and the reaction iscarried out in an aqueous solution.
 19. The method of claim 18, whereinthe nanoparticles remain suspended in the solution for at least 30 dayswithout substantial agglomeration and without substantial precipitationfrom the solution.
 20. The method of claim 10, further comprising:removing the nanoparticles from the solution; and placing thenanoparticles into an article.
 21. A method of making semiconductornanoparticles, comprising etching semiconductor nanoparticles of a firstsize to a second size smaller than the first size, wherein: the step ofetching comprises providing an etching liquid into an aqueous solutioncontaining the semiconductor nanoparticles of the first size; and thesolution contains a passivating element which binds to dangling bonds ona surface of the nanoparticles to passivate the surface of thenanoparticles.
 22. The method of claim 21, wherein the semiconductornanoparticles of the first size comprise crystalline semiconductornanoparticles.
 23. The method of claim 22, wherein the semiconductornanoparticles of the first size comprise metal sulfide, selenide ortelluride semiconductor nanoparticles.
 24. The method of claim 23,wherein: the etching liquid comprises hydrochloric acid.
 25. The methodof claim 23, wherein the nanoparticles remain suspended in the solutionfor at least 30 days without substantial agglomeration and withoutsubstantial precipitation from the solution.
 26. The method of claim 21,wherein the semiconductor nanoparticles comprise II-VI, III-V or IV-VIsemiconductor nanoparticles.
 27. The method of claim 21, wherein thesemiconductor nanoparticles of the first size have a random sizedistribution and the semiconductor nanoparticles of the second size havea uniform size distribution.
 28. The method of claim 21, wherein thesemiconductor nanoparticles of the second size have an average sizebetween 2 and 100 nm.
 29. The method of claim 28, further comprisingreacting at least a first sulfur containing reactant and a secondreactant in an aqueous solution to form the semiconductor nanoparticlesof the first size in the solution, wherein the first reactant provides asulfur passivating element which binds to dangling bonds on a surface ofthe nanoparticles to passivate the surface of the nanoparticles.
 30. Themethod of claim 21, further comprising placing the nanoparticles into anarticle.